1. Field of the Invention
The present invention relates to a dispersion compensating optical fiber, and to a dispersion compensating optical fiber module. This specification is based upon Japanese Patent Application 2000-359772, and hereby incorporates the contents thereof by reference.
2. Background Art
A dispersion compensating optical fiber is an optical fiber which compensates for chromatic dispersion which accumulates due to propagation through a single mode optical fiber which is incorporated in a transmission line of an optical communication system. In concrete terms, although for example chromatic dispersion occurs when a single mode optical fiber (such as one for use at 1.3 xcexcm) which has a zero dispersion wavelength on the short wavelength side of the 1.55 xcexcm band is used for this 1.55 xcexcm band, it is possible to compensate for this chromatic dispersion by utilizing a dispersion compensating optical fiber. As a result, it is possible to enhance the transmission speed of the optical communication system. In the following, for the convenience of description, the single mode optical fiber which is the object of compensation by this type of dispersion compensating optical fiber will be termed by the present inventors, xe2x80x98the single mode optical fiber for transmissionxe2x80x99.
On the other hand, in optical communication systems, widening of the wavelength band and increase of the number of multiplexed wavelengths is making rapid progress in accompaniment with the requirements for increase of capacity and increase of transmission speed.
In this connection, dispersion compensating optical fibers are also being developed which can compensate for chromatic dispersion of a single mode optical fiber for transmission in a wide wavelength band. For this it is necessary, not only to compensate for chromatic dispersion, but also to compensate for the dispersion slope which specifies the tendency of chromatic dispersion with respect to wavelength.
It should be understood that the chromatic dispersion and the dispersion slope in the 1.55 xcexcm band of a single mode optical fiber for transmission such as a single mode optical fiber for use at 1.3 xcexcm or the like are generally both positive values. Thus, a requirement for a dispersion compensating optical fiber is to have negative values of chromatic dispersion and dispersion slope.
A dispersion compensating optical fiber which has a so-called W-shaped refractive index profile, as shown in FIG. 1, has been proposed as one which can perform compensation for both chromatic dispersion and dispersion slope, as discussed above.
This dispersion compensating optical fiber with this refractive index profile is made up from a core 1 and a cladding layer 2 which is provided at an outer periphery of the core 1. And this core 1 is made up from a central core portion which is provided in its center and an intermediate core portion 4 which is provided at an outer periphery of this central core portion 3. The central core portion 3 is endowed with a higher refractive index than that of the cladding layer 2, while the intermediate core portion 4 is endowed with a lower refractive index than the cladding layer 2.
In FIG. 1, xcex941 is the relative refractive index difference of the central core portion 3 when the refractive index difference of the cladding layer 2 is taken as a reference. And xcex942 is the relative refractive index difference of the intermediate core portion 4 when the refractive index difference of the cladding layer 2 is taken as a reference. Further, a is the radius of the central core portion 3, while b is the radius of the intermediate core portion 4.
By adjusting the values of xcex941, xcex942, and b/a for a dispersion compensating optical fiber which has this W-shaped refractive index profile, characteristics are obtained which make it possible to compensate for the chromatic dispersion and the dispersion slope of a single mode optical fiber for transmission which has positive values of chromatic dispersion and dispersion slope, in for example the 1.55 xcexcm band.
Further, a dispersion compensating optical fiber which has a so-called segmented W-shaped refractive index profile as shown in FIG. 2 has also been developed, with the objective of increasing the effective area (hereinafter termed xe2x80x9cAeffxe2x80x9d), improving the bending loss, and widening the band of dispersion slope compensation.
This dispersion compensating optical fiber with this refractive index profile is made up from a core 11 and a cladding layer 12 which is provided at an outer periphery of the core 11. And this core 11 is made up from a central core portion which is provided in its center, an intermediate core portion 14 which is provided at an outer periphery of this central core portion 13, and a ring core portion 15 which is provided at an outer periphery of this intermediate core portion 14. The central core portion 13 is endowed with a higher refractive index than that of the cladding layer 12 and the intermediate core portion 14 is endowed with a lower refractive index than the cladding layer 12, while the ring core portion 15 is endowed with a refractive index which is lower than that of the central core portion 13 but is higher than that of the cladding layer 12.
In FIG. 2, xcex941 is the relative refractive index difference of the central core portion 13 when the refractive index difference of the cladding layer 12 is taken as a reference, xcex942 is the relative refractive index difference of the intermediate core portion 14 when the refractive index difference of the cladding layer 12 is taken as a reference, and xcex943 is the relative refractive index difference of the ring core portion 15 when the refractive index difference of the cladding layer 12 is taken as a reference. Further, a is the radius of the central core portion 13, b is the radius of the intermediate core portion 14, and c is the radius of the ring core portion 15.
In this case as well, by adjusting the values of xcex941, xcex942, xcex943, b/a, and c/b for a dispersion compensating optical fiber which has this segmented W-shaped refractive index profile, characteristics are obtained which make it possible to compensate for the chromatic dispersion and the dispersion slope of a single mode optical fiber for transmission which has positive values of chromatic dispersion and dispersion slope, in for example the 1.55 xcexcm band.
A dispersion compensating optical fiber may be, for example, incorporated into a cable which is inserted into a transmission line. Furthermore, it may be incorporated into a small sized dispersion compensating optical fiber module (sometimes hereinafter simply termed a xe2x80x9cmodulexe2x80x9d), which can be arranged at the receiving side or at the transmitting side of a transmission line which already exists. It should be understood that such a module may, for example, also incorporate a housing, within which there is stored a reel upon which the dispersion compensating optical fiber is wound. And within the housing the two ends of the dispersion compensating optical fiber are connected to single mode optical fibers for transmission which are for leading them out, and these optical fibers for leading out project to the outside of the housing through two lead out holes provided therein. Thus the dispersion compensating optical fiber module may be inserted into a transmission line by connecting a single mode optical fiber for transmission which is disposed externally to said housing to the end portions of these two optical fibers for leading out. Moreover, as a construction for this module, connection portions may also be provided upon the side surfaces of the housing for connecting the external single mode optical fiber for transmission.
By the way, it is desirable to obtain the benefit of compensation by using a comparatively short length of dispersion compensating optical fiber. In order to do this, it is desirable for the absolute value of the chromatic dispersion of the dispersion compensating optical fiber per unit length to be large.
Since a dispersion compensating optical fiber which is endowed with the above described W-shaped refractive index profile has a large absolute value of chromatic dispersion per unit length, so that Axcex941 is large, the radius of the core is set to a small value.
FIG. 3 is a graph showing the relationship between dispersion slope and chromatic dispersion for the W-shaped refractive index profile shown in FIG. 1, with the operating wavelength being 1.55 xcexcm, when Axcex941 was fixed at 1.8%, xcex942 was fixed at xe2x88x920.4%, and the values of b and b/a were changed. The wavelength for measurement was 1.55 xcexcm.
The broken line S in the graph shows the relationship between dispersion slope and chromatic dispersion when it was possible to perform 100% compensation for both chromatic dispersion and dispersion slope of a conventional single mode optical fiber for use at 1.3 xcexcm, and in the vicinity of this broken line S approximately ideal compensation operation was attained. Moreover, at an operating wavelength of 1.55 xcexcm, this single mode optical fiber for use at 1.3 xcexcm had a chromatic dispersion of +17 ps/nm/km and a dispersion slope of +0.055 ps/nm2/km.
In this graph, for each value of b/a, with the ratio b/a kept constant, a set of points was assembled as the value of b was changed by steps of 0.1 xcexcm. For each value of b/a, the range over which b was varied was as follows:
when b/a=3.0, b was varied from 4.0 xcexcm to 3.6 xcexcm;
when b/a=3.5, b was varied from 4.5 xcexcm to 4.2 xcexcm;
when b/a=4.0, b was varied from 5.0 xcexcm to 4.7 xcexcm;
when b/a=4.5, b was varied from 5.5 xcexcm to 5.3 xcexcm.
Since there was a tendency for the absolute value of the chromatic dispersion to become larger as the value of b became smaller, for each value of b/a, that value of b for which the absolute value of the chromatic dispersion was the largest was set as a lower limit value for the range of b, while that value of b for which the absolute value of the chromatic dispersion was the smallest was set as an upper limit value for the range of b.
Further, the numerical values in the graph are the values of Aeff. With an optical communication system, the transmission state is deteriorated when nonlinearity effects occur during the transmission of the optical signal, and it is known that this becomes an obstacle to long distance transmission and high speed transmission and the like. Nonlinearity effects can easily occur when the power of the optical signal is high. And in wavelength multiplex transmission the power of the optical signal is high, not only when amplifying the optical signal at relay points, but also from the start of input, and nonlinearity effects can easily occur.
And it is known that it is desirable for Aeff to be as large as possible, in order to inhibit nonlinearity effects.
Since as shown in the graph the value of Aeff for a dispersion compensating optical fiber which is endowed with a W-shaped refractive index profile is small, there has been insufficient inhibition of nonlinearity effects. Moreover, in the Proceeding of the 2000 Institute of Electronics, Information and Communication Engineers (IEICE) General Conference*1 etc., a dispersion compensating optical fiber which is endowed with a W-shaped refractive index profile is reported for which Aeff is 18.4 xcexcm2. However, no dispersion compensating optical fiber has ever been reported for which Aeff is yet greater.
*1: xe2x80x9cPerfectly dispersion slope compensated hybrid optical transmission line (PureCouple(trademark))xe2x80x9d C-3-38, P217, Proceeding of The 2000 Institute of Electronics, Information and Communication Engineers (IEICE) General Conference, published on Mar. 28, 2000. 
Further, with the segmented W-shaped refractive index profile shown in FIG. 2, according to past investigations by the present inventors, an optical compensating fiber with a value of Aeff of 21 xcexcm2 has been obtained, according to design conditions. However, the absolute value of chromatic dispersion for this dispersion compensating optical fiber has been the low value of 61.5 ps/nm/km, and the problem arises that the length which is required for compensating the chromatic dispersion of a single mode optical fiber for transmission is long. As a result the problem arises that the cost becomes high, since it often happens that the cost of the dispersion compensating optical fiber is high as compared with the cost of the single mode optical fiber for transmission.
Furthermore, the following types of problem particularly arise when manufacturing a module when the length which is used is long. Namely, when the dispersion compensating optical fiber is made into a module, it is necessary to be able to wind up the fiber upon a reel which is comprised in the module, and moreover to store the fiber in its state as wound up on the reel within the housing. Accordingly, when the length which is used is long, inevitably the reel and the housing become of considerable size, and along with the cost becoming high, the problem has arisen that the space required becomes great.
Yet further, although in this manner it is desirable from the points of view of space and of cost that the module should be designed to be as compact as possible, when the bending loss of the dispersion compensating optical fiber is high, the transmission loss becomes high when the fiber is wound up upon a small sized reel. However with a prior art type dispersion compensating optical fiber it has been considered as being extremely difficult, over a wide wavelength band, to satisfy the conditions that, along with being able to compensate the chromatic dispersion and the dispersion slope, Aeff should be large, the absolute value of the chromatic dispersion should be large, and moreover the bending loss should be low.
A primary object of the present invention is to provide a dispersion compensating optical fiber which is capable of compensating both the chromatic dispersion and the dispersion slope of a single mode optical fiber for transmission over as wide a wavelength band as possible, and for which Aeff is large and the absolute value of chromatic dispersion is large.
And another object of the present invention is to provide a dispersion compensating optical fiber which is able to suppress the occurrence of nonlinearity effects, so that the transmission loss is low.
A further object of the present invention is to provide a dispersion compensating optical fiber with which the length required (operating length) is short, so that the cost is reduced, and moreover so that it is possible to produce a small sized module which incorporates said compensating optical fiber.
Yet another object of the present invention is to provide a dispersion compensating optical fiber for which the bending loss is low, so that, in particular, little degradation of transmission loss is entailed, even when it is wound up upon a small sized reel which is received within a housing.
According to the present invention, it is possible to satisfy the above described objects by a dispersion compensating optical fiber which satisfies the conditions (1) through (4) below:
(1) it comprises a core and a cladding layer which is provided at an outer periphery of said core, with said core comprising a central core portion which has a refractive index greater than the refractive index of said cladding, an intermediate core portion which is provided at an outer periphery of said central core portion and which has a refractive index less than the refractive index of said cladding, and a ring core portion which is provided at an outer periphery of said intermediate core portion and which has a refractive index greater than that of said cladding;
(2) with the radii of said central core portion, said intermediate core portion and said ring core portion, and the relative refractive index differences with respect to the refractive index of said cladding layer being termed (a, xcex941), (b, xcex942) and (c, xcex943) respectively, then c is in the range of 6 to 9 xcexcm, xcex941 is in the range of 1.2% to 1.7%, xcex942 is in the range of xe2x88x920.25% to xe2x88x920.45%, xcex943 is in the range of 0.2% to 1.1%, b/a is in the range of 2.5 to 4.0, and c/b is in the range of 1.1 to 2.0;
(3) the effective area, at a wavelength band which is to be used (operating wavelength band) which is selected within the range of 1.53 xcexcm to 1.63 xcexcm, is greater than or equal to 20 xcexcm2; the bending loss at said operating wavelength band is less than or equal to 20 dB/m, the chromatic dispersion at said operating wavelength band is in the range of xe2x88x9270 to xe2x88x92100 ps/nm/km, and it possesses a cutoff wavelength at said operating wavelength band for which single mode propagation is effectively possible;
(4) when chromatic dispersion compensation is performed for a single mode optical fiber which has a zero dispersion wavelength which is shorter than said operating wavelength band by employing a length of this dispersion compensating optical fiber which can compensate the chromatic dispersion to zero, the compensation ratio of the dispersion slope is between 80% and 120%.
The dispersion compensating optical fiber according to the present invention is able to perform compensation for a single mode optical fiber for transmission both for chromatic dispersion and also for dispersion slope over a wide wavelength band, and moreover its value of Aeff is large, and the absolute value of its chromatic dispersion is large. As a result, desirable transmission characteristics are obtained due to the fact that it is possible to suppress the occurrence of nonlinearity effects, and furthermore, since the length thereof which is required is short, the cost is low, and moreover it is possible to incorporate it into a small sized module.
Yet further, it is particularly suitable for employment in a small sized module, since in this manner the length which is required is short and also the bending loss is low.